1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for processing seismic data to separate up-going and down-going wave fields recorded by a receiver underwater.
2. Discussion of the Background
In recent years, the interest in developing new oil and gas producing fields has dramatically increased. However, the supply of onshore production is limited. Thus, the industry has now extended drilling to offshore locations, which appear to hold a vast amount of fossil fuel. Offshore drilling is an expensive process. Thus, those engaged in such a costly undertaking invest substantially in geophysical surveys in order to more accurately decide where to drill in order to avoid a well with no or non-commercial quantities of hydrocarbons.
Marine seismic data acquisition and processing generate an image (2-dimensional cross section or 3-dimensional) of the geophysical structure (subsurface) under the seafloor. While this image/profile does not provide a precise location for oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of the oil and/or gas reservoirs. Thus, providing a high resolution image of the subsurface is an ongoing process for the exploration of natural resources, including, among others, oil and/or gas.
During a seismic gathering process, as shown in FIG. 1, a vessel 10 drags an array of detectors (e.g., hydrophones or geophones or accelerometers) 12. Plural detectors 12 are disposed along a cable 14. Cable 14 together with its corresponding detectors 12 are sometimes referred to, by those skilled in the art, as a streamer 16. The vessel 10 may tow plural streamers 16 at the same time. The streamers may be disposed horizontally, i.e., lying at a constant depth z1 relative to a surface 18 of the ocean. Also, the plural streamers 16 may form a constant angle (i.e., the streamers may be slanted) with respect to the surface of the ocean as disclosed in U.S. Pat. No. 4,992,992, the entire content of which is incorporated herein by reference. FIG. 2 shows such a configuration in which all the detectors 12 are provided along a slanted straight line 14 making a constant angle α with a reference horizontal line 30.
With reference to FIG. 1, the vessel 10 also drags a sound source 20 configured to generate a seismic wave 22a. The seismic wave 22a propagates downward and penetrates the seafloor 24, eventually being reflected by a reflecting structure 26 (reflector). The reflected seismic wave 22b propagates upwardly and is detected by detector 12. For simplicity, FIG. 1 shows only two paths 22a corresponding to the generated seismic wave. However, the seismic wave emitted by the source 20 may be substantially a spherical wave, e.g., it propagates in all directions starting from the source 20. Disturbances produced by the passing reflected seismic wave 22b (primary) are recorded by the various detectors 12 (the recorded signals are called traces) while disturbances produced by the reflected seismic wave 22c (reflected at the water surface 18) are detected by the detectors 12 at a later time. Since the interface between the water and air is well approximated as a quasi-perfect reflector (i.e., the water surface acts as a mirror for the acoustic or seismic waves), the reflected wave 22c travels back towards the detector 12 as shown by wave 22d in FIG. 1. Wave 22d is normally referred to as a ghost wave because this wave is due to a spurious reflection. The ghosts are also recorded by the detector 12, but with a different polarization and a time lag relative to the primary wave 22b. 
Thus, every arrival of a marine seismic wave at detector 12 is accompanied by a ghost reflection. In other words, ghost arrivals trail their primary arrival and are generated when an upward traveling wave is recorded a first time on submerged equipment before being reflected at the surface-air contact. The now downward propagating reflected wave 22d is recorded a second time at detector 12 and constitutes the ghost. Primary and ghost (receiver side ghost and not the source side ghost) signals are also commonly referred to as up-going and down-going wave fields.
The time delay between an event and its ghost depends entirely upon the depth of the receiver 12 and the wave velocity in water (this can be measured and considered to be approximately 1500 m/s). It can be only a few milliseconds for towed streamer data (depths of less than 15 meters) or up to hundreds of milliseconds for deep Ocean Bottom Cable (OBC) and Ocean Bottom Node (OBN) acquisitions. The degenerative effect that the ghost arrival has on seismic bandwidth and resolution are known. In essence, interference between primary and ghost arrivals causes notches or gaps in the frequency content and these notches cannot be removed without the combined use of advanced acquisition and processing techniques.
One popular technique for separating the up-going and down-going wave fields is called PZ-summation and applies to both OBC/OBN and streamer data. Here, the seismic wave field is recorded using co-located hydrophones (P) and vertical geophones (Z). In other words, the detector 12 shown in FIG. 1 includes two different devices, the hydrophone 32 and the vertically oriented geophone 34. Hydrophones measure pressure whereas geophones measure particle velocity in the direction they are oriented. Data recorded on both receivers is in phase for up-going waves and of opposite phase for down-going waves, or the ghost. Combining both records involves a calibration to remove differences in frequency response, a unit conversion (which depends on the impedance, defined as the product of water density and wave velocity, of the water) and a time-offset dependant scaling to match amplitudes. After these steps the data can be summed or subtracted to produce estimates of the up-going and down-going wave fields respectively.
However, each of the above corrections (spectral matching, unit conversion, and time-offset scaling) has to be estimated and all are prone to errors.
Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks, e.g., remove interference and improve the usable band width of the data that can be used in subsequent analysis, such as obtaining a final image.